Bituminous paint not curing on chassis

I’m restoring a Mitsubishi GTO and I’ve been removing rust from its chassis, then zinc priming, epoxy coating and finally in areas where stones do hit, coating with bituminous coating (3M Schulz to be precise).

In certain areas there was no rust and thus I didn’t remove the original coating (don’t know what it is exactly). In those areas when I sprayed with bituminous over it, it never cured. 3 months later it hasn’t cured and I’ve contacted 3M to ask for advice to which they said that I can try heat to see if it will cure. Heat helped a little bit (made it less tacky) but didn’t fix the problem. This isn’t an issue where I used epoxy mastic and then sprayed with 3M Schulz, in those areas the coating has cured and isn’t tacky.

I don’t know what to do next, one option is obviously to remove it with the grinder, and then prime it, epoxy coat it and then apply Schulz again. Is there another option? Can I remove it with a thinner and then coat the underside with epoxy rather than going back to bare metal?

Has anyone got any experience with bituminous coatings reacting with previous chassis coatings?